A white, 17-year-old police admirer has been arrested after two people were shot to death during a third straight night of protests in Kenosha, Wisconsin, over the police shooting of African American man Jacob Blake.
Kyle Rittenhouse, of Antioch, Illinois, about 25km from Kenosha, was taken into custody in Illinois on suspicion of first-degree intentional homicide in Tuesday's attack that was largely captured on mobile phone video.
The shooting left a third person wounded.
"I just killed somebody," the gunman, carrying a semi-automatic rifle, could be heard saying at one point during the rampage that erupted just before midnight in the city of 100,000 people midway between Milwaukee and Chicago.
In the wake of the killings, Wisconsin Governor Tony Evers authorised the deployment of 500 members of the National Guard to Kenosha, doubling the number of troops.
"A senseless tragedy like this cannot happen again," the governor, a Democrat, said in a statement.
In Washington, the Justice Department said it is sending in more than 200 federal agents from the FBI and US Marshals Service in response to the unrest.
The White House said up to 2000 National Guard troops would be made available.
The dead were identified only as a 26-year-old Silver Lake, Wisconsin, resident and a 36-year-old from Kenosha. The wounded person, a 36-year-old from West Allis, Wisconsin, was expected to survive, police said.
"We were all chanting 'Black lives matter' at the gas station and then we heard, boom, boom, and I told my friend, "That's not fireworks,"' 19-year-old protester Devin Scott told the Chicago Tribune.
"And then this guy with this huge gun runs by us in the middle of the street and people are yelling, 'He shot someone! He shot someone!' And everyone is trying to fight the guy, chasing him, and then he started shooting again."
According to witness accounts and video footage, police apparently let the gunman walk past them and leave the scene with a rifle over his shoulder and his hands in the air as members of the crowd were yelling for him to be arrested because he had shot people.
Sheriff David Beth described a chaotic, high-stress scene, with lots of radio traffic and people screaming, chanting and running - conditions he said can cause "tunnel vision" among law officers.
Much of Rittenhouse's Facebook page is devoted to praising law enforcement, with references to Blue Lives Matter, a movement that supports police.
In a photograph posted by his mother, he is wearing what appears to be a blue law enforcement uniform as well as the kind of brimmed hat that state troopers wear.
The sheriff told the Milwaukee Journal Sentinel that militia members or armed vigilantes had been patrolling Kenosha's streets in recent nights, but he did not know if the gunman was among them.
However, video taken before the shooting shows police tossing bottled water from an armoured vehicle to what appear to be armed civilians walking the streets. And one of them appears to be the gunman.
"We appreciate you being here," an officer is heard saying to the group over a loudspeaker.
Blake, 29, was shot in the back seven times on Sunday as he leaned into his SUV, three of his children seated inside.
On Wednesday state authorities identified the officer who shot Blake as Rusten Sheskey, a seven-year veteran of the Kenosha Police Department.
State agents later recovered a knife from the driver's side floorboard of the vehicle, authorities said.
On Tuesday, Ben Crump, the lawyer for Blake's family, said it would "take a miracle" for Blake to walk again.